There have been various vigorous studies on a non-aqueous electrolyte secondary battery including lithium or a lithium compound as the negative electrode, because it is to be expected to offer a high voltage as well as a high energy.
To date, oxides and chalcogens of transition metals like LiMn.sub.2 O.sub.4, LiCoO.sub.2, LiNiO.sub.2, V.sub.2 O.sub.5, Cr.sub.2 O.sub.5, MnO.sub.2, TiS.sub.2, MoS.sub.2 and the like are known positive electrode active materials for non-aqueous electrolyte secondary batteries. Those compounds have a layered or tunneled structure that allows free intercalation and deintercalation of lithium ions. On the other hand, there are many previous studies on metallic lithium as the negative electrode active material. However, metallic lithium has a drawback that a deposition of lithium dendrites occurs on the surface of the electrode during charging, which reduces charge/discharge efficiency or causes internal short-circuiting due to contact between formed lithium dendrites and the positive electrode. As one measure for solving such drawback, the use of a lithium alloy such as lithium-aluminum alloy which not only suppresses the growth of lithium dendrites but also can absorb therein and desorb therefrom lithium as the negative electrode has been under investigation. However, the use of such lithium alloy has a drawback that repeated charge/discharge operation causes pulverization of the electrode, which in turn deteriorates the cycle life characteristics. At present, lithium ion batteries have been put into practical use that include as the negative electrode a graphite-based carbon material having excellent cycle life characteristics and safety capable of reversibly absorbing and desorbing lithium although smaller in capacity than the above-mentioned negative electrode active materials.
When the above-mentioned graphite material is used in a negative electrode, the practical capacity used thereof is 350 mAh/g which is a value near the theoretical capacity (372 mAh/g). Since the theoretical density is as low as 2.2 g/cc and the density further decreases when the graphite material is formed into a negative electrode sheet, use of metallic materials having higher capacity per volume as the negative electrode is desired.
However, problems arising when metallic materials are used as the negative electrode include pulverization caused by repeated expansion and contraction accompanying intercalation and deintercalation of lithium. Due to this pulverization, the reactivity of the active material lowers and charge/discharge cycle life shortens.
For solving these problems, there has been for example a suggestion for solving pulverization, intending to suppress expansion by means of stress relaxation of a phase not absorbing lithium even under charged condition (absorbing condition) in coexistence in one particle of a phase absorbing lithium and a phase not absorbing lithium (Japanese Laid-Open Patent Publication (JP-A) No. 11-86854). Further, there has been a suggestion in which two or more phases absorbing lithium are allowed to exist in one particle, intending to relax expansion due to change in structure during absorbing lithium of each phase for suppression of pulverization (JP-A No. 11-86853).
However, even a negative electrode material produced by utilizing these means causes pulverization of the active material along with progress of a charge/discharge cycle, thereby to increase cycle deterioration. The reason for this is hypothesized as follows. Namely, when a plurality of phases are present in an active material particle, even if releasing of expansion stress into the interface of phases is possible, non-uniformity in stress tends to occur in an active material particle along with increase in expansion coefficient of each phase. Therefore, pulverization occurs from some phases on which expansion stress is applied significantly, and this pulverized material liberates from the active material particle. Thus, pulverization of the active material progresses. When one phase is composed solely of an element easily forming an alloy with lithium, the pulverization as described above tends to occur more easily.
The object of the present invention is to provide a negative electrode for non-aqueous electrolyte secondary batteries having high electric capacity and excellent charge/discharge cycle life characteristics, in view of the above-described drawbacks.
The present invention provides a negative electrode for non-aqueous electrolyte secondary batteries satisfying high electric capacity and long cycle life at the same time by preventing pulverization accompanying expansion and contraction.